Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes an electrode lead implanted at the desired stimulation site and an implantable pulse generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the electrode lead or indirectly to the electrode lead via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. A typical stimulation parameter set may include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation current at any given time, as well as the amplitude, duration, rate, and burst rate of the stimulation pulses.
The neurostimulation system may further comprise a handheld remote control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes using one or more current-controlled sources for providing stimulation pulses of a specified and known current (i.e., current regulated output pulses), or one or more voltage-controlled sources for providing stimulation pulses of a specified and known voltage (i.e., voltage regulated output pulses). The circuitry of the neurostimulator may also include voltage converters, power regulators, output coupling capacitors, and other elements as needed to produce constant voltage or constant current stimulus pulses. Conventional battery-operated neurostimulators typically apply stimulation pulses to the tissue that are referenced to an internal circuit voltage in the neurostimulator, with a relatively low impedance connection being located between one or more stimulation electrodes and internal circuitry. This relatively low impedance effectively clamps the voltage on these stimulation electrodes to the internal circuit voltage.
For example, a voltage source can be coupled between the internal circuitry and an anode to create a cathode clamped voltage regulated circuit (FIG. 1a), a current source can be coupled between the internal circuitry and an anode to create a cathode clamped current regulated circuit (FIG. 1b), a voltage source can be coupled between the internal circuitry and a cathode to create an anode clamped voltage regulated circuit (FIG. 1c), and a current source can be coupled between the internal circuitry and a cathode to create an anode clamped current regulated circuit (FIG. 1d). It can be appreciated that the reference voltage will be at the cathodes for the topologies illustrated in FIGS. 1a and 1b and will be at the anodes for the topologies illustrated in FIGS. 1c and 1d. 
Because the voltage at the unregulated side of the electrode will be clamped to the voltage of the internal circuitry, and because the stimulation output circuitry may be unbalanced in that some components in the circuitry (coupling capacitors, protection circuits, etc.) may be present on the cathode side of the circuit but not the anode side of the circuit, or vice versa, the output stimulation circuitry between the cathode and the anode will be asymmetrical, such that the cathode and the anode will be asymmetrically referenced to the internal circuit. For example, a shift in voltage in the output stimulation circuit results in asymmetrical voltage shifts between the anodes and cathodes.
In particular, the voltage of the common mode signal (i.e., the average of the anode voltage shift and cathode voltage shift relative to the reference voltage) will be equal to or greater than the differential voltage between the cathode and anode. For example, as shown in FIG. 2a, when the cathode voltage is at the internal reference voltage, the common mode signal is equal to one-half the differential voltage between the cathode and anode. As shown in FIG. 2b, when the cathode voltage is above the internal reference voltage, the voltage of the common mode signal is greater than one-half the differential voltage between the cathode and anode. As shown in FIG. 2c, when the cathode voltage is below the internal reference voltage, the voltage of the common mode signal is likewise greater than one-half the differential voltage between the cathode and anode. The asymmetry between anodes and cathodes in the output stimulation circuitry may be associated with undesired side effects during stimulation that lead to reduced patient comfort. In particular, parasitic coupling of the common mode signal to the implantable device can give rise to an additional stimulation signal that is superimposed on the differential stimulation signal. Even if the common mode signal is subthreshold by itself, it can modulate the differential stimulation signal, causing unwanted activation of neural tissue.
In addition to the problem of asymmetry in the output stimulation circuit, referencing the voltage at the cathodes and anodes to an internal circuit may require excessive voltage levels at the cathodes and anodes in order to maintain the desired voltage potential therebetween. For example, if the desired voltage potential between a cathode and an anode is 5V, and if the internal voltage is 20V, the voltage at the anode would have to be 25V and the voltage at the cathode would have to be 20V. The increased voltage at the electrodes will increase the voltage relative to the tissue, which may cause problems such as unwanted stimulation and even electro-chemical reactions resulting in corrosion of the electrodes.
There, thus, remains a need for an improved method and system for conveying stimulation to tissue in a controlled manner.